1. Field of the Invention
The present invention relates to a method of examining distance data calculated based on stereoscopic images as well as to an examination device for performing such examination.
2. Description of the Related Art
Considerable attention is given in recent years to outside view monitoring systems using a stereo camera system as a preview sensor. This kind of monitoring system takes pictures of views in front of own vehicle with a pair of onboard cameras (stereo camera system) fitted to a vehicle body and calculates distance data based on a pair of images obtained using the principle of triangulation. Then, based on the calculated distance data, the monitoring system recognizes traffic conditions ahead of own vehicle, and call a driver""s attention as the need arises or performs vehicle behavior control operation such as deceleration by a downshift.
When this kind of stereo camera system is fitted to the vehicle body, a high degree of accuracy is required with respect to its mounting position. This is because an error in the mounting position of the stereo camera system results in a deviation of the shooting direction of the stereo camera system, directly affecting calculated distance data. Besides this, there are cases where the reliability of the calculated distance data decreases due to unbalanced sensitivity of the stereo camera system, the influence of camera lens properties, or improper image corrections, for instance.
Thus, it is necessary to calculate distance data from stereoscopic images obtained by actually taking pictures with a stereo camera system and then examine the results of calculation in a testing process to be carried out upon completion of the installation of the stereo camera system. Conventionally, however, examination technique of this kind has not been established and, therefore, it has strongly been desired to establish examination technique which would make it possible to achieve improved efficiency and automation of examination.
This invention has been made in consideration of the aforementioned circumstances. Accordingly, it is an object of the invention to provide an examination method which makes it possible efficiently and accurately examine the reliability of distance data calculated based on stereoscopic images.
To solve the aforementioned problems, a first aspect of the invention provides a method of examining the reliability of distance data calculated based on stereoscopic images, the method of examining the distance data comprising the steps of obtaining a pair of photographed images by taking pictures of a test chart carrying a brightness pattern with a stereo camera, the test chart being placed in a shooting direction of the stereo camera, calculating the distance data from the two photographed images, producing a histogram showing a relationship between distance values of evaluation samples and their frequency of occurrence by adopting the distance data concerning the test chart shown in the photographed images as the evaluation samples, and judging the reliability of the calculated distance data by evaluating properties of the histogram thus produced based on the distance between the stereo camera and the test chart.
Here, it is preferable that the aforementioned step of judging be a step which judges the reliability of the calculated distance data based on the ratio of the frequency of occurrence of the evaluation samples included in an appropriate range set in the histogram to the total number of the evaluation samples. This appropriate range is a specific distance range including the distance between the stereo camera and the test chart.
The aforementioned step of judging may be a step which judges the reliability of the calculated distance data based on whether a peak distance value concerning the frequency of occurrence which forms a peak in the histogram exists within an appropriate range. This appropriate range is a specific distance range including the distance between the stereo camera and the test chart.
This peak distance value may be a distance value whose frequency of occurrence is at a maximum. Alternatively, the peak distance value may be calculated based on a distance value whose frequency of occurrence is at a maximum or a distance value adjacent to the distance value whose frequency of occurrence is at a maximum, whichever gives a higher frequency of occurrence. Furthermore, it is preferable to use a distance value calculated in sub-pixel units.
The aforementioned step of producing the histogram may be a step which adopts distance data derived from each of multiple evaluation sub-areas set in the photographed images as the evaluation samples and produces histograms showing a relationship between the distance values of the evaluation samples and their frequency of occurrence. In this case, the aforementioned step judges whether peak distance values in peak portions, in which the frequency of occurrence reaches its peak, exist within the appropriate range for each of the histograms, and further judges that calculation results of the distance data are good when all of the peak distance values have been judged to exist within the appropriate range.
An examination device of a second invention for examining the reliability of distance data calculated by a stereoscopic outside view monitoring system comprises a stereo camera which is fitted to a vehicle body and outputs a pair of photographed images by taking pictures of conditions ahead of the vehicle, stereo processing means for calculating the distance data from the pair of photographed images, and examination means which judges the reliability of the distance data based on the photographed images obtained by taking pictures of a test chart carrying a brightness pattern with the stereo camera, the test chart being placed at a predetermined position ahead of the vehicle during examination. This examination means adopts distance data derived from an evaluation area which is provided at a central part of each of the photographed images as evaluation samples, produces a histogram showing a relationship between distance values of the evaluation samples and their frequency of occurrence, and judges the reliability of the calculated distance data by evaluating properties of the histogram thus produced based on the distance between the stereo camera and the test chart.
Here, the examination means may judge the reliability of the calculated distance data based on the ratio of the frequency of occurrence of the evaluation samples included in an appropriate range set in the histogram to the total number of the evaluation samples. This appropriate range is a specific distance range including the distance between the stereo camera and the test chart.
The examination means may judge the reliability of the calculated distance data based on whether a peak distance value concerning the frequency of occurrence which forms a peak in the histogram exists within an appropriate range. This appropriate range is a specific distance range including the distance between the stereo camera and the test chart.
Furthermore, the examination means may adopt distance data derived from each of multiple evaluation sub-areas set in the photographed images as the evaluation samples and produce histograms showing a relationship between the distance values of the evaluation samples and their frequency of occurrence. In this case, the examination means judges whether peak distance values in peak portions, in which the frequency of occurrence reaches its peak, exist within the appropriate range for each of the histograms, and further judges that calculation results of the distance data are good when all of the peak distance values have been judged to exist within the appropriate range.
In the first and second inventions, it is preferable to use a brightness pattern having a configuration in which blocks of the same shape but varying brightness gradations are arranged at random.